The fusing of xerographic copies by a hot roll fuser, i.e. a hot roll in pressure contact with a cold or cool roll, is of course well known. In such a fuser the hot roll is maintained at an elevated temperature, sufficient to melt or soften the toner, thus causing the toner to adhere to the copy paper.
It has been found that unsightly toner patterns or tracks are sometimes formed when sheets are supplied to a straight-line fusing nip, i.e. the nip formed by two circular cylinders. While the cause of these tracks is not known, it is believed that the sheet sometimes fails to pass through the nip in a planar fashion, i.e. the portion of the sheet which has not entered the nip distorts, causing premature contact with the hot roll, thereby smearing the unfused toner image. Gross distortions, which may occur for example at relative humidities above 60%, may also cause wrinkling of the copy paper.
It has been found that this adverse phenomenon, associated with passing copy paper through a fusing nip formed by a circular cylinder hot roll in pressure contact with a circular cylinder cold backup roll, can be solved by providing a an orientation means and method whereby the sheet's initial physical contact to the fusing nip is essentially point contact, rather than the usual contact which comprises the sheet's entire leading edge, as has been done in the prior art. More specifically, the present invention provides a skewed approach orientation of a planar sheet to the fusing nip, and/or a non-planar, i.e. bowed, orientation approach of the sheet to the fusing nip. With the structure of the present invention, the sheet's initial contact occurs at a point. With a skewed approach, this point is a leading corner of the sheet. With a bowed approach a single point, usually near the center of the leading edge, first contacts the surface of the backup roll, and then moves into the fusing nip.
The present invention has been found to have particular utility where the fusing nip is formed by a resilient hot roll and a non-resilient backup or cold roll.
The general concept of guiding a sheet to the fusing nip is, or course, well known. In the art of hot roll fusing, it is also known that creasing of the sheet, as it passes through the fusing nip, is reduced by the use of a rigid, concave-hollow cylindrical hot roll, and a complementary shaped resilient pressure or backup roll.
In the art of liquid developing electrophotography it is known that the transfer step may be improved by guiding the sheet to the transfer station such that the sheet's leading edge describes a curve when initially contacting a drum photoconductor.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.